A capstan can be seen as a black box acting as a force amplifier or a force reducer for a rope, a wire, or any elongated member running therethrough. In the following the word “rope” shall be understood as including any elongated member adapted to be connected to and to carry a load. The force amplification or reduction through a capstan will follow the Eytelwein formula:
            S      1              S      2        ≤      e    μα  wherein S1 is the rope force acting on a high tension side of the capstan, S2 is the rope force acting on a low tension side of the capstan, μ is the friction coefficient, and α is the total angle swept by all turns of the rope measured in radians. The maximum rope force amplification or reduction will be reached when
            S      1              S      2        ≤      e    μα  and the rope will start sliding. In the following the S1/S2 ratio will be called the amplification factor or reduction factor, depending on the direction of the travel of the rope. Because sliding of the rope is undesirable, capstan systems are typically overdesigned by letting the rope sweep one or more turns in addition to what is needed. The one or more additional turns will create more bends for the rope running through the capstan. Usually this is considered to be a small price to pay for having gained additional safety against sliding. For a load smaller than the maximum load of the system, the capstan will be even further overdesigned, doing several unnecessary turns around the capstan sheaves. Usually, this is also considered to be a small price to pay for gaining a high safety factor against sliding.
Overdesigning a capstan system, however, creates a number of drawbacks, some of which are not understood by most capstan designers, and which might even reduce the safety of the system. The drawbacks that are described in the following will typically be even more pronounced in offshore lifting devices comprising heave compensation systems for counteracting the heave acting on a vessel on which the lifting device is placed. The rope will be travelling back and forth in order to compensate for the heave motion of the vessel to keep the load stable relative to a seabed or to another vessel, and the rope will thus be exposed to a great number of bending cycles with the rope under high tension.
In a capstan system sheaves are typically arranged quite close to each other. The sheaves might be connected, such as in a double capstan system, or they might be individually driven, or the capstan might comprise a combination of the connected and individually driven sheave arrangements. If the motion of the rope is frequently reversed, such as in an above mentioned offshore heave compensated operation, this might lead to a premature failure of the rope.
When a rope is travelling back and forth in a capstan, the affected rope sections will be bent every time they enter a sheave and they will be straightened every time they leave a sheave. Due to the friction within the rope and between the rope and the sheaves, the affected rope sections and capstan sheaves will heat up. This might lead to a loss of lubricant, and as a consequence, to an accelerated degradation of the rope. Heat might also start a strain ageing process in the rope wires. The heating effects tend to increase with increasing rope diameter.
When a rope does more bending cycles than necessary, there is a risk that exponential force amplifications build up from both sides of the capstan, thus leading to a force peak in the capstan system. The rope travelling through the capstan then not only does more bending cycles than necessary but these bending cycles might be done under loads higher than what is considered to be a maximum rope force of the system. The combination of the unfortunate effects of excessive bending cycles and loads will lead to a much higher bending fatigue and, as a consequence, to a considerably reduced rope life. In several cases the rope force have increased to a level higher than the breaking strengths of the ropes, which have thus lead to overload failures of the ropes within the capstan system. Some failures of capstan orientation/axes might also be the consequence of the fact that the rope forces in the system were much higher than what the designer had predicted.
Yet another drawback of the prior art arises from the fact that the force distribution in a capstan system is different when a load is lifted compared to when a load is lowered. This implies that every time the motion of the rope is reversed, the force on a section of a rope may be substantially increased or lowered. This will lead to a great amount of tension-tension fatigue which will add up to the already mentioned bending fatigue due to the superfluous sheaves. In addition the rope will continuously change its length while lying on a sheave in order to adapt to the changing rope forces, thus causing abrasion both on the rope and on the sheaves of the capstan.
The drawbacks of the prior art will also be described below with reference to the accompanying drawings.